Need Help on Tube Rolling for VAC Avatar

Just bought my very first tube amp (integrated), VAC Avatar, a couple months ago and it has been great. Pairing with the Sonus Faber Grand Piano speakers and Rega Planet 2000 CD player, with Nordost Blue Heaven IC and bi-wired speaker cables, I'm quite happy with the current set up.

HOWEVER, like any other self respected 'goner here, I just can't stay status quo and am contemplating of experiment tube rolling, with hopes to explore even better sound (I know it gets very personal here). The problem is...I don't know where and how to start and would like to get some advice from you guys.

First off, here are the 3 sets of stock tubes (Chinese) that I have in the amp:
- 12AX7 x 2 (line stage)
- 12AU7 x 3 (power amp)
- EL34 x 4 (power amp)

Newbie Questions:
- for the most bang for the buck, which set should I try rolling first?
- any suggestions as to what brand gives you what type of sound relative to others? (e.g. Mullard gives you a lush sound and much warmer relative to RCA clear top?!?)
- before I have to spend the money that I don't have, what is considered "entry level" tubes good for experimenting?
- comments on the following brands: Mullard, Telefunken, RCA, Svetlana, Sylvian, Svotek, etc.
- what's a good and reliable place to buy tubes? Used? NOS?
- perspective on Chinese vs. Russian tubes?

As you can tell, I have no clue where and how to start but would love to learn more. That said, the last thing I want to do is to spend hundreds of $ and get nothing accomplished. Any advice is greatly appreciated. Thanks in advance, guys.

Steve